Not applicable.
Not applicable.
The present invention relates to headboxes used in forming a fibrous web in general and to headboxes and forming sections of pulp formersin particular.
In the economy of papermaking the most basic asset is standing timber which can be harvested and made into paper. The process of turning standing timber into paper has many steps including harvesting the timber, reducing the logs harvested to wood chips, processing the wood chips to produce wood fiber, forming a paper sheet from the wood fibers, and finally finishing the paper by coating and calendaring the finished web. The value of the product formed increases as it is further processed. A given quantity of standing timber is of less value than logs ready for chipping, wood chips are less valuable than processed pulp fibers, and pulp fibers are less valuable than finished paper. At each stage of the process, value is added. As greater value is added to the natural resourcexe2x80x94the standing timberxe2x80x94a greater investment in capital equipment is required. In many economies, the basic natural resources consumed are of relatively small value compared to the value added through labor and capital. In areas which are less developed in terms of the amount of capital available and the skills of the workforce, but which have access to plentiful natural resources, the economy may be more based on the harvesting and exporting of resources, such as timber.
The natural path of development for economies which have significant timber resources is to begin by exporting timber, and, through investment over time, to progress to the export of paper. This process is facilitated by a world economy which creates markets for intermediate products, from raw logs to wood chips, to wood pulp, to finished paper. Even within a developed economy which processes standing timber to finished paper, economies may dictate the buying and selling of the intermediate products between producers so that particular entities may specialize in buying logs and selling chips, or buying chips and selling pulp.
The manufacturing process for pulp requires that the pulp be handled as a suspension in water. Typically pulp for use in paper production will be contained in a stock containing about 96 percent water by weight. In papermaking, the stock is diluted further to about 99 percent water by weight and formed into a paper web by injecting the stock through a headbox onto a wire. A papermaking machine is a large capital intensive machine which must often be reconfigured between significantly different paper grades. If pulp is to be sold before it is manufactured into a paper web, either to avoid the capital cost of a papermaking machine or because the economics of a particular supplier dictates that the wood fibers be sold as fibers, the wood fibers must be concentrated to reduce the shipping cost associated with the water in which they are suspended. A specialized machine known as a pulp former has been developed which can remove approximately 95 percent of the water contained in a four percent fiber content stock, to produce a mat of fibers which is only 50 to 55 percent water. Further drying the mat is generally uneconomical because heated dryers would be required.
Unlike papermaking machines, which generally use very diluted stock, a pulp former operates with a pulp stock containing about four percent fiber dry weight. The unique properties of a pulp former present considerably different design problems and design solutions compared to a papermaking machine. In particular, a pulp former headbox is designed to keep the high fiber content stock fluid so that a uniform sheet can be formed between a pair of forming wires and uniformly dewatered on the forming table and through a series of press nips. The pulp former typically operates with forming wire speeds which are less than about five percent of the forming speed of a typical papermaking machine.
Existing pulp formers have been developed which use flexible stainless-steel blades which extend in the machine direction and the cross machine direction and extend from the lips of a pulp former slice into tangential engagement with that twin wires of a forming section of the pulp former. The flexible sealing blades extend just slightly past the tangent point of the forming rolls which support the forming wires of the pulp former. Pressure supplied by the pulp former headbox seals the flexible blades against the forming wires, preventing a pulp leak between the headbox of the forming wires which can result in an uneven web which presents problems in the pressing section of the pulp former. In known pulp formers, the flexible sealing blades have been sandwiched between a holder on the pulp side of the blades and the sealing blade supports which fix the blades to the headbox. Most recently, in order to avoid flow transitions, the use of a holder has been eliminated and the blades have been welded directly to the sealing blade supports. However welding the blades to the sealing blade supports presents manufacturing difficulties which can result in the blades warping due to the heat of welding. If the blades become warped than the pulp pressure may not be sufficient to seal the blades against the forming wires and thus the pulp may leak between the wires and the forming blades.
What is needed is a better way of mounting the sealing blades in a pulp former to the sealing blade supports which mount the sealing blades to the pulp former slice.
The pulp former of this invention employs a headbox wherein the sealing blades which extend between the headbox slice and the twin forming wires are bonded to blade supports which mount the blades to the headbox slice. The blade supports have flat mounting surfaces which extend in the machine and cross machine directions and which overlie or underlie the blades supports. An adhesive film between the flexible stainless-steel blades affixes the blades to the supports which are in turn mounted to the upper and lower sides of a pulp former headbox slice. After the adhesive bond is formed, holes are drilled through the flexible blades and into the blade supports. The holes are filled with pins which are mounted flush to the pulp side surfaces of the flexible blades. The pins serve to additionally supply shear resistance between the blades and blade supports. The flexible stainless-steel blades are relatively thin being approximately 1 to 1xc2xd mm thick. The holes through the flexible blades may be tapered, and the corresponding tapered portions of the pins may be used to clamp the flexible blades to the blade supports.
It is an object of the present invention to provide a more reliable method for attaching flexible blades to the blades supports attached to the slice of a pulp former headbox.
It is another object of the present invention to provide a pulp forming headbox with more securely and uniformly attached flexible forming blades.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.